This invention relates to a two cycle engine and more particularly to a two cycle engine as utilized in conjunction with an outboard motor.
As is well known, two cycle engines have a great versatility and wide application due to their extremely compact nature and the simplicity in their construction. In addition, such engines normally produce a high output for a given displacement due to the fact that the power cycle occurs during every crankshaft revolution as opposed to a four cycle engine wherein the engine fires and supplies power only every other crankshaft revolution. In order to increase the performance of such engines, and particularly to improve the scavenging efficiency, it has been the practice to provide porting arrangements wherein there is a substantial overlap between the opening of the intake port and the closing of the exhaust port. Such high overlaps provide good scavenging and high performance. However, at low engine speeds the port overlap can cause rough running and otherwise poor performance.
As a result, the port timing has been a compromise between good running at low speeds and maximum performance. In order to avoid these compromises, it has been proposed to employ an exhaust control valve in the exhaust port for controlling both the back pressure and the timing of the opening of the exhaust port. Various arrangements have been incorporated for controlling the timing of the exhaust port and the exhaust gas pressures through the manipulation of this valve.
In addition, it has also been proposed to incorporate a decompression device for facilitating starting in two cycle engines and running at idle or lower speed. Such decompression devices normally open the combustion chamber to the exhaust port during at least a portion of the operation so as to assist in starting, particularly by hand, by reducing the cranking pressure. Of course, such decompression ports should be closed during normal and particularly high speed running so as to avoid a loss in power output.
The opening of the decompression valve and the opening of the valve controlling the exhaust port also can significantly effect the exhaust noises generated by the engine. Obviously, the greater the amount of exhaust gasses flowing into the exhaust system and also the opening of the decompression port can significantly increase the exhaust noise. Therefore, the control of these valves also is dictated in part by the silencing system for the engine and the amount of noise which must be suppressed.
Although the use of decompression in exhaust port valves is fairly well known in two cycle engines, the application of these principles has, for the most part, not been applied to outboard motors. One reason for this is that an outboard motor, because of its unique exhaust system, presents different problems than those encountered with other applications of two cycle internal combustion engines. In conjunction with an outboard motor, it is the normal practice to discharge the exhaust gases through an underwater exhaust gas discharge. In this way, the body of water in which the outboard motor is operating can be used as a silencing device for the exhaust gases. With such underwater exhaust gas discharges, the depth of submersion of the discharge varies on the speed of the associated watercraft and frequently above the water exhaust gas discharges are employed for operating at low speeds.
It is, therefore, a principal object of this invention to provide an improved arrangement for controlling the exhaust port in an outboard motor embodying a two cycle internal combustion engine.
It is a further object of this invention to provide an arrangement for controlling the flow of exhaust gases through the exhaust system of an outboard motor so as to improve performance and reduce sound.
It is a further object of this invention to provide an improved decompression system for an outboard motor.